Cathodes emit electrons when elevated in temperature by a process known as thermionic emission. Thermionic emitters generally consist of a wire that is made of some refractory metal, which may be typically made of tungsten or molybdenum. The wire is then coated, or impregnated with some low work function material, such as barium carbonate, and subsequently ohmically heated.
Hollow cathodes have been in existence for over ten years. Hollow cathodes have been developed to an advanced state of technology readiness for ion propulsion. Ionic propulsion may be defined as propulsion by the reactive thrust of a high-speed beam of similarly charged ions ejected by an ion engine. In ground tests they have demonstrated high emission currents of greater than 30 Amperes, and long lifetimes, with modest power requirements of less than 100 Watts. Hollow cathode plasma sources have demonstrated versatile and effective operation as plasma contactors in ground testing of various devices. This testing includes plasma bridge neutralizers for ion thrusters, plasma contactor demonstration experiments for the electrodynamic tether, and space station structure potential control experiments.
Hollow cathodes have also been flown in space as components of ion propulsion systems and spacecraft charging/charge-control systems, including ATS-6, SERT-II, SCATHA, and SCSR-1 flight experiments. Demonstrated capabilities in space tests include lifetimes of 10,000 hours and more than 300 restarts. NASA flight experiments have demonstrated hollow cathode plasma contactors to be effective in controlling both the negative charging and differential charging of the spacecraft frame. Hollow cathodes have been operated in space under a variety of orbital and environmental conditions; on spacecraft, including an Agena vehicle, on communication satellites, and on the space shuttle. Environments include those of low-earth orbits, sun-synchronous high inclination orbits, and geosynchronous orbits.
All of the above hollow cathode development was accomplished with mercury as the hollow cathode expellant, or "working fluid." For a variety of reasons, which includes spacecraft contamination, the present hollow cathodes preferentially use an inert gas, such as xenon, as the expellant. Subsequent to the transition from the use of mercury to xenon in the early 1980's, there have been, and continue to be, failures of hollow cathodes in the United States, in Europe, and in Japan. These have impacted both research and development activities and flight programs. The failures have apparently been primarily due to inadequate procedures and protocols to control contamination during the fabrication, assembly, testing, storage, handling, and operation of the cathodes; as well as, inadequate design and process features. To date, the only successful extended duration tests, that have been reported of using inert-gas hollow cathodes at high emission currents of greater than 1 Ampere, have been conducted, by the NASA Lewis Research Center. These successful extended duration tests were implemented by the use of the design features and processes that are further described herein.
U.S. Pat. No. 3,944,873, granted Mar. 16, 1976, to J. Franks, et al., discloses a hollow cathode of cylindrical shape. A cathode encloses an anode having a pair of screen electrodes, symmetrically disposed about and parallel to the plane of the anode. The anode has a central aperture and another aperture may be made in the cathode diametrically opposite the first aperture.
U.S. Pat. No. 4,049,989, granted Sep. 20, 1977, to R. H. Bullis, et al., discloses ion production using a permeable electrode having apertures and a central electrode. A wire mesh grid is placed symmetrically about the permeable electrode.
U.S. Pat. No. 4,087,721, granted May 2, 1978, to G. Mourier, discloses an ion source that is comprised of a hollow cathode discharge arrangement having an anode placed between two cathodes. The cathode has holes through which some of the ions of the plasma escape.
U.S. Pat. No. 4,377,773, granted Mar. 22, 1983. to A. Hershcovitch. et al., discloses an ion source that is comprised of a hollow cathode and an anode base having electrically connected anode covers.
U.S. Pat. No. 4,428,901, granted Jan. 31, 1984, to W. H. Bennett discloses a hollow cathode, that is held inside of a cathode holder, as well as, a hollow anode that is supported by a conducting support. A diode envelope surrounds the hollow cathode.
U.S. Pat. No. 4,894,546, granted Jan. 16, 1990, to R. Fukui et al., discloses a cylindrical hollow cathode having upper and lower circular anodes that are placed at the two ends of the cylindrical cathode, where each of the anodes have circular openings.
U.S. Pat. No. 5,075,594, granted Dec. 24, 1991, to R. W. Schumacher, et al., discloses a hollow cathode used for discharging ionized plasma of an ambient gas, such as xenon. A flat anode extends perpendicular to, and is intersected by, the axis of the cathode. A keeper/baffle electrode, which may also be a plate, is disposed between the cathode and anode. Even though this device is a low impedance device, it will not yield electron emission currents to an external electrode in the multi-ampere range, within a voltage range of 20 Volts.
U.S. Pat. No. 5,241,243, granted Aug. 31, 1993, to G. Cirri, discloses a plasma generator that is comprised of a hollow cylindrical cathode and one or more anodes.
U.S. Pat. No. 5,352,954, granted Oct. 4, 1994, to G. Cirri, discloses a plasma generator that is comprised of a hollow cylindrical cathode and one or more anodes having holes.
U.S. Pat. No. 5,569,976, granted Oct. 29, 1996, to N. V. Gavrilov, et al., discloses of an ion emitter that is comprised of a hollow cathode at one end and a coaxial rod-shaped anode at the other end. The hollow cathode encloses the rod shaped anode.
U.S. Pat. No. 5,581,155, granted Dec. 3, 1996, to A. I. Morozov, et al., discloses a plasma accelerator that is comprised of a hollow cathode and an annular anode.
All of the above referenced prior art relate to high voltage acceleration systems. Further, they do not teach of a self-regulating emission control system. None of the prior art relates to an ionic emission apparatus, having low current capability, with the exception of U.S. Pat. Nos. 5,075,594 and 4,428,901, which disclose the use of electron emission apparatus. Only U.S. Pat. No. 5,075,594, teaches of a low output impedance, whereas all of the others have an undesirable high output impedance, that is not suitable for use in space station applications.
In addition, none of the above referenced prior art provide an attained performance reliability having a demonstrated lifetime in excess of the present state-of-the-art 500 hours, when operated at emission currents of approximately 1 Ampere.
These devices in the past have exhibited unstable operating characteristics and shortened lifetimes as a result of design and processing problems. Until the initiation of the present program, there have been no inert gas hollow cathodes that had demonstrated lifetimes greater than 500 hours, when operated at emission currents greater than 1 Ampere.
The present invention differs from the aforementioned prior art inasmuch that the approach is not limited solely by the design of the apparatus but also includes the method of manufacturing processes and procedure in order to obtain a highly reliable and repeatable design commensurate with a high life expectancy. The advancements demonstrated in the manufacturing processing include contamination control procedures which cover hollow cathode component cleaning procedures, gas feed system designs and specifications, and hollow cathode activation and operating procedures.
Accordingly, it is therefore an object of the present invention to provide an electron emissive hollow cathode apparatus that will provide reliable, stable and repeatable operation over a lifetime that is in excess of 17,500 hours.
It is another an object of the present invention to provide an electron emissive hollow cathode apparatus that will provide reliable, stable and repeatable operation over a broad range of operating emission currents of at least a 6:1 ratio.
It is still another an object of the present invention to provide an electron emissive hollow cathode apparatus that will provide reliable, stable and repeatable operation, while permitting electron emission currents of up to 30 Amperes emission to an external anode, at voltages of less than 20 Volts DC.
It is a final object of the present invention to provide a method of manufacturing an electron emissive hollow cathode apparatus that when adhered to, will provide reliable, stable and repeatable operation over an expected lifetime that is in excess of 17,500 hours.
These as well as other objects and advantages of the present invention will be better appreciated and understood upon reading the following detailed description of the presently preferred embodiment taking in conjunction with the accompanying drawings.